1. Field of the Invention
The present invention relates to injection molding devices, and more specifically to a friction puller of a multiple plate injection mold.
2. Discussion of Related Art
Friction pullers are used in injection mold application to help control the sequential movements of various plates during mold operations. One such friction puller is shown in German Patent No. DE 2815698, which is assigned to the HASCO Company. Friction pullers made according HASCO have a bolt which has a substantially conical shaped shank. The bolt is inserted into a resin jacket that has a corresponding substantially conical shaped passage for receiving the bolt shank. When the bolt is tightened, the bolt head compresses the jacket lengthwise and the shank presses laterally on the jacket. The result is that under the load from the tightened bolt, the jacket has an increased diameter compared to when it is in an unloaded condition.
Another type of friction puller is also produced by HASCO. In these friction pullers, a bolt with a straight set of threads is used. The bolt is secured into a key member that passes through one of the mold plates. The key has at least one flat surface to prevent the key from rotating inside the mold plate. The end of the key that receives that bolt has a hemispherical wedge section integral to or fixed to the key member. A resin jacket is again used. The jacket has a hemispherical cup inside one end of the jacket to correspond to the hemispherical wedge on the key member. The other end of the jacket has a second hemispherical cup for receiving a hemispherical washer used between the bolt head and the jacket. Again, when the bolt is tightened, the hemispherical washer and hemispherical wedge compress the jacket lengthwise forcing the jacket to increase slightly in diameter.
A problem encountered with both of these types of friction pullers is that they do not accommodate typical manufacturing conditions with respect to installation or operational environments, which results in inconsistent performance. For example, these products are fix mounted into the mold plates. Thermal expansion, build tolerances and the like can lead to the pullers not being centered with respect to the respective pockets in the opposing mold plates. While small, these misalignments can cause the mold to lock because of different force levels exerted by each of the friction pullers in the mold. Typically, multiple friction pullers are used in a single mold.
Another problem encountered with both of the types of friction pullers described above is the creation of air pressure differentials during use. As the pullers are designed to friction fit into the pockets that receive them, often the puller creates an air tight seal against the pocket. During mold close, the mold has to compress the air in the pocket to close the mold. This compressed air wants to push the puller out of the pocket. Alternatively, if the air escapes-the pocket during mold closure, but the jacket creates an air tight seal once the mold is fully closed, a vacuum is created in the pocket when the puller attempts to pull out from the pocket. Neither of the friction pullers described above provide for air to flow in and out of the pocket during mold open an mold closure operations. Accordingly, either the compressed air or vacuum conditions can occur randomly, creating an undesirable variation in the force exerted by the friction pullers.
Thus, there remains a need for an improved friction puller. There remains a need for a self-aligning friction puller. There remains a need for a friction puller to controllably allow air to pass into and out of the pocket that receives the friction puller.